This invention relates to a valve tappet for an internal-combustion engine and, more particularly, to a hydraulic valve tappet for an internal-combustion engine, having a housing which is disposed in a receiving device and having a piston which is displaceably guided in this housing. The piston is connected with a charge cycle valve.
Tappets of this type are used for the transmission of forces acting between the camshaft and the intake and the exhaust valves. In modern high-performance internal-combustion engines, cup-shaped valve tappets are frequently used which are disposed in the cylinder head to be directly displaceable between the cams and the valves. These are usually connected to the oil circulating system and cause a hydraulic valve clearance compensation which renders respective servicing operations superfluous, as known, for example, from the German Patent Document DE-37 24 655.
For improving the torque, the emission, the fuel consumption and the idling quality, it is known to design the valve stroke and/or the valve overlap to be variable. A cam-controlled valve tappet which influences the valve stroke using a hydraulic arrangement is disclosed, for example, in the German Patent Document DE-36 25 627. The tappet shown there is in an operative connection with a charge cycle valve by way of a hydraulic space. For the variation of the valve stroke, a portion of the hydraulic fluid can be discharged from the hydraulic space by a control valve.
A survey of variable valve actuating mechanisms is disclosed in SAE Paper 891674, "A Survey of Variable Valve Actuation Technology".
From the journal Automotive Engineering, Volume 91, Number 11, 1983, Pages 61-66, it is known to construct a valve, through which an electroviscous fluid (EVF) flows, of several cylinder shells which are arranged coaxially with respect to one another and which are spaced with respect to one another by radially extending struts. This valve is inserted as a piston into a hydraulic cylinder through which an EVF flows. This fluid flows axially through the cylinder shells in an unimpaired manner so that the piston remains in its position. By the feeding of an electric field, the viscosity of the EVF can be changed arbitrarily within a very short time period from liquid to solid. In this case, the flow resistance between the cylinder shells increases so that the piston is displaced by the afterflow of fluid into the hydraulic cylinder.
From the German Patent Document DE-36 09 861, it is known to utilize the two electrodes and the EVF-layer disposed in-between as a movement sensor for the control and regulation of a hydraulic EVF-system (Oppermann effect). As a function of the flow rate of the EVF between the electrodes, a flow signal is generated in the electrodes which is fed to an electronic circuit which supplies a corresponding output voltage to the electrodes and therefore, in turn, affects the viscosity. The movement sensor therefore, at the same time, represents the control element for influencing the movement.
There is therefore needed a hydraulic valve tappet for an internal-combustion engine by means of which the valve stroke can be varied during the operation of the internal-combustion engine.
This need is met by a hydraulic valve tappet for an internal-combustion engine, having a housing which is disposed in a receiving device and having a piston which is displaceably guided in this housing. The piston is connected with a charge cycle valve. A rigid hydraulic high-pressure valve is fastened in the housing. A variable operating space is formed between the high-pressure valve and the piston, which is filled with a fluid. The viscosity of the fluid can be changed by the feeding of an electric voltage.
This valve tappet permits a continuous variation of the valve stroke of a charge cycle valve by the arrangement of a rigid high-pressure valve, that is, a high-pressure valve which has no moving parts, and of an operating space which is formed between this high-pressure valve and the displaceable piston and is filled with a fluid the viscosity of which can be changed by the feeding of an electric voltage.
Such an electroviscous fluid (EVF) can be changed with respect to its viscosity by means of the electric voltage from "liquid" to "hard". A voltage which is fed to the high-pressure valve builds up an electric field which penetrates the fluid and causes it to solidify. For varying the valve stroke, the fluid, when a cam moves onto the valve tappet, is at first liquid so that the piston is displaced by the transmitted forces and in the process fluid is displaced out of the operating space through the high-pressure valve. When the desired valve stroke is reached, the fluid solidifies into a firm medium and, because of the short switching times for the viscosity change (milliseconds), the now rigid connection between the cam and the piston opens the charge cycle valve.
In an advantageous development, the displaced fluid is pushed into a compensating space formed between the high-pressure valve and a housing bottom which is acted upon by the cam, so that the fluid will remain inside the valve tappet; its volume is therefore low. When the cam moves off the housing bottom, the viscosity is changed to "liquid", and a spring arranged between the housing bottom and a sealing disk covering the compensation space displaces the fluid through the high-pressure valve back into the operating space.
The remaining fluid inside the valve tappet avoids the problems which normally occur in the case of oil-hydraulic tappets as a result of foamed oil, such as tappet rattling and resulting noises which simulate combustion noises at or above the knock limit to a knock control.
The required electronic voltage is transmitted in a no-contact manner by induction windings inserted into the valve tappet housing and into a tappet guide. An electronic control device controls the electric voltage and thus the valve stroke. Because of the Oppermann effect, the tappet is self-regulating since, when the fluid is displaced through the cylinder shells, a voltage is induced which is a function of the displacement rate and which transmits the actual valve stroke to the switching circuit. For certain parameters of the internal-combustion engine, such as the load, the rotational speed, and the oil temperature, optimal valve strokes are stored in characteristic diagrams in the control device. Thus, as a function of the parameter, a valve stroke can be implemented in each case which is optimal, for example, with respect to a minimal emission of pollutants or a maximal performance.
The control may take place individually for each charge cycle valve. In the case of an internal-combustion engine equipped with, for example, two intake valves per cylinder, the two valves can open successively with different strokes in phase-shifted manner. Furthermore, for a low, consumption-optimized power range of the internal-combustion engine, one of the two intake valves may be disconnected in that the adjusting path of the piston in the housing is selected to be as long as the maximal cam pitch, and the viscosity at the point of the maximal cam pitch is adjusted to "solid".
The tappet can be used, for example, as a cup tappet in a displaceable manner directly between the cam and the charge cycle valve, or it may be used as a stationarily disposed tappet. In the case of the stationary arrangement, for example, as a base for a rocker lever or rocker arm, the tappet guide and the induction windings will not be necessary because, in this case, the high-pressure valve is connected directly to the electronic control device.
Irrespectively, the valve tappet causes an automatic valve clearance compensation in every case, for example, in the case of a positive control. This results in lower manufacturing and servicing expenditures.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.